1. Field of the Invention
The present invention is directed to an olefin polymerization catalyst system. More particularly, the present invention is directed to an olefin polymerization system which provides improved catalytic activity and permits the synthesis of polymers having improved processability.
2. Background of the Prior Art
The continual development of new applications of olefin polymers has necessitated the development of new processes, employing new catalyst systems, to improve productivity in this highly competitive market. Thus, there is a continuing need to improve catalyst components and catalyst systems to improve their activity as well as to provide improved olefin polymers.
The development of new ethylene polymers, for example, linear low density polyethylene, has been very much welcomed in the marketplace. The applications to which linear low density polyethylene is put continues apace. The development of linear low density polyethylene, however, has not been free of difficulties. Processability of linear low density polyethylene and other ethylene polymer products remains a concern. That is, the high degree of polymerization, as manifested by ethylene polymer melt index, makes difficult the processing of these polymers.
Yet another objective in the development of catalyst systems is manufacturing olefin polymers, especially ethylene polymers, having narrow molecular weight distribution. This property is manifested by melt index ratio (MIR). The MIR, quantitatively determined as the ratio of melt index under high load to melt index under normal load, is desirably low to ensure a narrow molecular weight distribution.
Various catalyst components and catalyst systems have been developed to provide one or more of these properties. It is obvious, however, that the development of a new catalyst system which provides all of the aforementioned goals would be very much welcomed.
U.S. Pat. No. 4,349,648 describes a catalyst composition which comprises a precursor composition formed by dissolving at least one titanium compound, at least one magnesium compound and at least one electron donor. Although the titanium-containing compound can be a multiplicity of compounds, the magnesium compound is limited to a magnesium dihalide of which magnesium chloride is most preferred. The electron donor may be an alkyl ester of an aliphatic or an aromatic carboxylic acid, an aliphatic ether, a cyclic ether or an aliphatic ketone. The precursor composition is activated by contact with an organoaluminum compound. The '648 patent indicates that a mixture of a dihydrocarbylaluminum halide and a trihydrocarbylaluminum present in the molar ratio of at least 4:1 but not more than 20:1 produces copolymers processable into films having improved optical properties. More preferably, this molar ratio of dihydrocarbylaluminum halide compound to trihydrocarbylaluminum compound is in the range of from 5:1 to 15:1.
U.S. Pat. No. 4,833,111 discloses a silica supported catalyst employed in the polymerization of olefins. In the formation of this supported catalyst component, an organomagnesium compound, preferably dibutyl magnesium, is disposed upon the silica support followed by contact with an alkanol, preferably ethanol. The thus prepared composition is thereupon contacted with a transition metal compound which is usually a vanadium or a titanium compound. Most preferably this transition metal compound is titanium tetrachloride. The product of this contacting step is, in turn, contacted with an alkyl aluminum halide. The preferred alkyl aluminum halides include ethylaluminum dichloride or diethylaluminum chloride. The thus formed catalyst composition is activated with trimethyl aluminum.
U.S. Pat. No. 4,981,929 sets forth a catalyst system for the production of highly stereospecific olefin polymers. The olefin polymerization catalyst system disclosed in the '929 patent includes a solid titanium-containing procatalyst, an organoaluminum compound activator and a selectivity controlling agent. The solid titanium-containing procatalyst is obtained by reacting a magnesium compound having the formula MgR.sub.1 R.sub.2, where R.sub.1 is alkoxy or aryloxy; R.sub.2 is alkoxy, aryloxy or halogen, with a halide of a tetravalent titanium compound in the presence of a halohydrocarbon and an electron donor in the liquid phase. The thus formed halogenated reaction product is thereupon contacted with a tetravalent titanium compound.
The organoaluminum activator compound of the '929 catalyst system may be triethylaluminum. The selectivity control agent of that system preferably is ethyl p-ethoxy benzoate or p-ethyl anisate. To this catalyst system is added a dialkylaluminum halide. Stoichiometric aspects of the catalyst system include an aluminum, in the organoaluminum activator compound, to titanium, in the cocatalyst, molar ratio in the range of 40:1 and 100:1. The molar ratio of dialkyaluminum halide to titanium is in the range of 5:1 to 25:1.
U.S. Pat. No. 4,888,318 teaches a catalyst system for the polymerization of olefins. The catalyst system includes a catalyst component and a cocatalyst activator. The catalyst component includes an inert carrier, preferably silica, dried at elevated temperature, followed by contact with an aluminum or zinc alkyl compound. The resultant product is contacted with a magnesium-containing compound, preferably magnesium dichloride. Thereupon the titanium compound, preferably titanium trichloride, is reacted with the thus formed product. Finally, an electron donor, which also acts as the solvent in the preparation mixture, is added. The catalyst system of the '318 patent also includes a cocatalyst component, trimethylaluminum. Of particular interest are three examples included therein as Example 4 which utilized mixtures of diethylaluminum chloride followed by addition of tri-n-hexyl aluminum to partially activate the catalyst component.
U.S. Pat. No. 4,378,304 comprise an olefin polymerization catalyst system in which a catalyst component is formed by disposing a dialkyl magnesium compound on a silica or an alumina support. The thus formed product is contacted with water or alcohol such that the molar ratio of water or alcohol to magnesium is in the range of between about 0.8 and 1.0. The water- or alcohol-contacted product is contacted with a transition metal compound of Group IVB and/or VB of the Periodic Table. In a preferred embodiment the transition metal compound is titanium tetrachloride, providing a titanium to magnesium molar ratio of about 0.8 to 1.0. The thus formed catalyst component is combined with a cocatalyst which may be one of many aluminum-containing compounds of which triisobutylaluminum is preferred. However, the cocatalyst may also be diethylaluminum chloride.
U.S. Pat. No. 4,564,647 discloses an ethylene polymerization catalyst comprising (A) a contact treatment product of (a) a high activity catalyst component and (b) a filler; and (B) an organoaluminum compound. The '647 patent also describes a further embodiment of the catalyst which includes, as a third component, (C) a filler having an affinity, equal to or higher than the filler (b), for polyethylene.
The high activity catalyst component (a) includes a hydrocarbon soluble transition metal compound, which preferably is a tetravalent titanium compound, reacted with a fatty acid salt, a alcoholic salt or a long chain aliphatic hydrocarbon group-containing phosphoric acid salt of magnesium or manganese. Alternatively, the high activity catalyst component (a) can be a cyclopentadienyl compound having the formula (Cp)MR.sup.1 R.sup.2 R.sup.3, where Cp is a cyclopentadienyl group; M is titanium, vanadium, zirconium or hafnium; and R.sup.1, R.sup.2 and R.sup.3 are independently C.sub.1 -C.sub.6 alkyl, a cyclopentadienyl group, halogen or hydrogen.
The filler (b) is chosen depending upon the polyethylene composition desired. The filler may be a metal, a metal oxide, a metal carbonate, a carbonaceous substance, a ceramic, an organic pigment or a solid foaming agent.
Component (B) is an organoaluminum compound which may be one or more compounds having the general formula R'.sub.m AlX'.sub.3-m, where R' is C.sub.1 -C.sub.6 alkyl, a cycloalkyl or an aryl; X' is halogen; and m is a positive real number of 3 or less, specifically 1, 1.5, 2 or 3. Examples given of organoaluminum compounds include trialkylaluminum compounds and dialkylaluminum monohalides. A mixture of triethylaluminum and diethylaluminum chloride is given to exemplify a cocatalyst within the scope of the '647 patent.